The present invention relates to a display system for a liquid crystal display (LCD) device, and more particularly to a method and a mechanism for expanding the number of gray scale levels in the LCD device.
There has been a tendency in recent years to immediately imagine color display when a liquid crystal display (LCD) device is considered. Actually, with regard to an LCD module used for an LCD monitor or the like, one using a so-called 8-bit/color source driver for displaying red (R), green (G) and blue (B) by 8-bit data has been widespread. Such an LCD module can perform multi-gray scale display of 28=256 levels for one color. For the entire colors of R. G and B, (28)3=16 M (approximately 16 million) colors can be displayed.
On the other hand, color display is not always necessary for the use of the display. Monochrome display may be enough or even better. There may also be a case where higher resolution and more gray scale levels are requested. A display device for medical use is a typical example. For such a special purpose, a CRT monitor capable of performing monochrome display with high resolution and many gray scale levels has conventionally been used. In general, such a monochrome CRT monitor can receive and display 12-bit data from a graphics adapter of a host system, i.e., data capable of displaying gray scale of 212 levels. Accordingly, the LCD display must have a capability of displaying gray scale of the same levels.
The market of monochrome monitors is very attractive to LCD module/monitor manufacturers, and it is only natural that they consider the possibility of making a monochrome LCD monitor the replace for a monochrome CRT monitor. Nowadays, especially in LCD monitors having ultra high resolutions, such as QXGA (Quad Extended Graphics Array) (2048×1536 dots), or the QUXGA (Quad Ultra Extended Graphics Array) (3200×2400 dots), the limitation of CRTs can be greatly exceeded in a pixel pitch. For example, in the case of a 20.8 inch LCD monitor based on QXGA, pixel pitches are as follows:    Horizontal: (⅘) 20.8 25.4/2048=0.20637    Vertical: (⅗) 20.8 25.4/1536=0.20637The pitches are about 206 micrometers for both horizontal and vertical lines. These pitches for character display are too fine to human eyes (best pixel pitches for character display are considered to be about 300 micrometers), but 206 micrometers is considered to be a value suitable for graphics display.
Thus, there are no problems for the use of a LCD monitor in high resolution requirements. However, there exists a great problem in the number of gray scale levels to be displayed. Specifically, for example, in the case of the monochrome LCD monitor, unless 212 or more gray scale levels are provided, the replacement of the CRT monitor by the LCD monitor may lose its attraction to the user, and may even be abandoned. Accordingly, it is an important task to realize the monochrome LCD monitor having many gray scale levels while providing lower costs.
Conventionally, as measures to provide more gray scale levels by a display device having the equal number of data bits, a dither method and Frame Rate Control (FRC) have been widely used.
The dither method is spatial modulation in short, which is designed to realize gray scale levels of 2n or more seemingly by, for example, entering data of n+2 bits from a host system to a display device originally having the data bit number of n bits, and performing spatial modulation for the original gray scale value of its pixel represented with upper n bits by using lower 2 bits.
Another method called FRC is time modulation in short, which is designed to seemingly increase the number of gray scale levels by performing modulation for each frame in this case (i.e., adding +1 or −1 to its original gray scale value) by using bits also expanded to the lower side.
It is possible to use the dither method and the FRC in combination. For example, Japanese Patent Laid-Open No. Hei 3-39717 discloses a technology for performing multi-gray scale display by dividing each display pixel of a liquid crystal display device into four portions, and then increasing the number of display gray scale levels thereof when each display pixel is displayed. A similar technology for using the dither method and the FRC is also disclosed in Japanese Patent Laid-Open No. Hei 6-301357.
However, the use of the foregoing dither method requires the sacrifice of resolution to increase gray scale. Consequently, it is impossible to attain a high resolution. The use of the FRC causes a difference in luminance between frames, and flickering on the screen becomes conspicuous depending on a displayed pattern, resulting in degradation of image quality. In addition, the foregoing disclosed technology is designed only to combine the dither method and the FRC and, thus, the above problems still remain to be solved. A practical rate of increase made in the number of gray scale levels by employing the dither method and the FRC is only about 22 to 23. Even in the case of the display device of 8-bit/color, the total number of gray scale levels is limited to at most 210 to 211, which is far less than the number 212 (=4096) of gray scale levels presented by the currently used monochrome CRT.
Now, consideration is given to the case of performing monochrome display by simply removing a color filter (e.g., omitting a color filer generation process) from a generally used color thin-film transistor (TFT) LCD panel and the like. In this case, original three pixels corresponding to R, G and B can be considered to be one pixel of monochrome display. In the case of 8-bit color, while the gray scale values of these three subpixels are increased from (m, m, m) to (m+1, m+1, m+1) (0≦m≦28−1), two luminance levels of (m, m+1, m+1) and (m, m, m+1) can be employed. At this time, (m, m, m+1), (m, m+1, m) and (m+1, m, m) are considered to have equal luminance levels, and thus these cannot be distinguished from each other. The same applies to (m, m+1, m+1), (m+1, m, m+1) and (m+1, m+1, m). As a result, the number of gray scale levels to be displayed is 3 (28)−2=766.
The foregoing content will be further described. It is assumed that the luminance of each of portions originally called R, G and B is N. The sum total of these luminances is 3N. Preconditions are that each portion can be displayed by 8-bit/color, i.e., by 28=256 gray scale levels, and that a gamma characteristic between luminance and a gray scale value can be represented by a linear function, assuming that black is 0. In this case, the gray scale levels of R, G and B can be respectively represented in 0, N/255, 2N/255, . . . and 255N/255. By combining R, G and B, display can be made at the gray scales of 0, N/255, 2N/255, . . . and 765N/255. Accordingly, the number of gray scale levels becomes 766. In other words, even in the case of monochrome display realized by removing the color filter from the color LCD panel, the total number of gray scale levels using the display device of 8-bit/color is far less than 210. 